Light-metal alloy



atented Nov 16, 1926.

UNITED- STATES PATENT OFF-ICE.

HERBERT 11., Dow AND JOHN A. GANN, or MIDLAND, MICHIGAN, ASSIGNORS TO THE Dow CHEMICAL COMPANY, OF MIDLAND, MICHIGAN, A CORPORATION OF'MIGHI- GAN.

No Drawing.

The present improved alloy is one in vhich magnesium is the predominant element, such .metal being preferably present 11 excess of 80 per cent, as prescribed 1n 3. S. Patent No. 965,485, to Pistor et al., lated July 26, 1910. As an additional 1nrredient, or one'of the additional Ingrelients, where a plurality of other metals 1s ldded, we employ zinc in a relativelysmall amount, as'hereinafter set forth.

The addition of zinc to magnesium 1n )rder to make a light metal alloy of the class in question has indeed been heretofore described in U. S. Patent No. 982,022 to Riibel, dated January 17 1911, the stated proportions of the zinc being not less than 1 per cent nor more than 6 per cent, and in the more recent U. S. Patent No. 1,341,774, to Backer, dated June 1, 1920, a more complex alloy is disclosed consisting of notless than 85 per cent and not more than 98 per cent of magnesium, not less than 1 per cent and not more than 14 per cent of aluminum, and not less than of 1 per cent nor more than 6 per cent of zinc. However, as a result of an extensive investigation of these known magnesium-zinc alloys and numerous carefully conducted tests, both of a scientific character and under conditions of practical use, we have discovered that even the minimum proportions of zinc prescribed by Riibel and Backer. respectively, are too high to produce an alloy that will successfully withstand an elevated temperature under conditions of strain whether induced by pressure or otherwise. Specifically p'is tons for internal combustion engines, when made of such alloy having a content of even 0.75 per cent of zinc, show a tendency to collapse'when used in engines where high compression is employed with corresponding increase in operating temperatures. The presence of zinc in solid solution, nevertheless, is highly desirable since it contributes materially to the strength and toughness of the alloy, the effect being proportional to the zinc content, and measurably increases the wearing quality of an article such as a pis- LIGHT-METAL ALLOY.

Application filed December 12 1921. Serial No. 521,695.

ton, in the use of which Wear is a very important factor.

Our invention, accordingly, rests on the discovery that the desirable results accruing fromthe use of zinc in an alloy of the type under'consideration may be obtained by a reduction in the proportion of such zinc below that heretofore realized as possible and at the same time an alloy is produced that will successfully withstand relatively elevated temperatures, thus adapting the alloy for use in gas engines where high compression is employed. Even in the case of engines normally operating at a lower temperature than where such high compression is employed, such engines are of course always liable, through faulty lubrication or an accident to the cooling system, to become overheated, and so our present improved alloy is of general utility for the construcion of pistons for such engines, and the Zield of IS use is of course not limited mere- 1*; to gas engine pistons, but such alloy will l-e found superior to known alloys in various connections where similar conditions of use are imposed. The present invention, accordingly, consists of a light metal alloy in which magnesium is the predominant ingredient, as aforesaid, such alloy containing in addition a limited amount of zinc with or without the addition of further ingredients,

as hereinafter fully described and particularly pointed out in the claims, it being understood that the several specific formulas herein given are merely illustrative of the various possible combinations of such alloying ingredients that may be utilized in carrying out the principle of our invention.

It should be noted, in the first place, that zinc when alloyed with magnesium does two independent and distinct things, viz, (1 a part of such zinc dissolves in the magneslum adding both toughness and tensile strength thereto; and (2) another part does not thus dissolve but combines witha small part of the magnesium to form a eutectic structure.-

Such magnesium-zinc eutectic, however, has a relatively very low meltlng point, namely,

approximately 650 F., which is below the temperatures frequently encountered in the operation of internal combustion or gas englnes especially where the latter are operated at high compression. To the fusion of such eutectic we attribute the collapse of pistons formed of a magnesium-zinc alloy since we have found that by reducing the zinc content to a point where no such eutectic is formed but only the solid solution, this diificulty is entirely overcome. The exact point at which such magnesiumzinc eutectic begins to form, in the scale of percentage composition by weight, is probably between 0.75 and 1.00 per cent of zinc; and in order to avoid with certainty its formation we preferably employ not to exceed 0.65 per cent of zinc, and in certain formulas only 0.50 per cent of zinc; while even a smaller proportion (for example, 0.30 per cent) of zinc results in a decided increase in strength and toughness, although it is obviously desirable to use maximum safe limit.

A eutectic structure is, nevertheless, desirable in an alloy of the class under consideration imparting, as it does, an element of rigidity to the metal somewhat akin to that produced by the steel rods, etc, in reinforced concrete. We accordingly alloy one or more ingredients in addition to such zinc with the magnesium, such ingredients forming a eutectic with a melting point safely above the temperature at which the alloy is to be subjected in service. It is a matter for secondary consideration whether such added ingredient or ingredients also partly go-into solid solution with the magnesium; thus, for example, in our preferred formulas we employ 8 per cent or a trifle more of aluminum which is capable both of going into solid solution with the magnesiumand also of forming a relatively high melting point eutectic. Such magnesium-aluminum eutectic, in other words, has a melting point of approximately 824 R, which isnot so low as to be objectionable. Copper may'similarly be added in an amount in excess of the zinc content, such addition being entirely safely made, since the magnesium-copper'eutectic has a melting polnt of approximately 900 F. Also, because of certain desirable qualities which it adds to the alloy, we may employ a small percentage of cadmium which does not form any eutectic at all but goes entirely into solid solution withthe magnesium; or in place of such cadmium, approximately the same amount of tin may be substituted.

The following formulas are given by. way of example of combinations of ingredients that have been actually tested out with entire success in the construction of pistons for internal combustion engines, where such (4) Magnesium 88.00 per cent; aluminum 8.50 per cent; copper 2.00 per cent; cadmium 1.00 per cent; and Zinc 0.50 per cent.

, Where copper is employed as an ingredient, it is always present in the eutectic and, as stated above, cadmium is always present in solid solution. It will further be understood tl.1t'the point up to which zinc is present in solid solution will vary somewhat, depending upon what other constituents are employed inaddition thereto. Furthermore, therate of cooling of the casting has an effect on the percentage that remains in solid solution and so it is desirable that the highest limit possible be not approached as otherwise there is danger that some of the low melting magnesium-zinc eutectic will be formed. We, therefore, prefer, as a matter of safety, to keep the zinc content definitely below 0.75 per cent, as indicated in the illustrative formulas given above, in which 0.65 per cent of zinc is the highest used.

Other modes of applying the principle of our invention may be employed instead of the one explained, change being made as regards the materials employed, provided the ingredients stated by any ofthe following claims or the equivalent of such stated ingredients be employed.

We therefore particularly point out and distinctly claim as ourinvention 1. As a new product, an alloy containing magnesium and zinc, wherein. the magnesium largely predominates and the zinc is all in solid solution, the amount of such zinc being less than 0.50 per cent.

2. As a new product, an alloy containing magnesium and zinc, wherein the magnesium largely predominates and the zinc is all in solid solution, said alloy also containing one or more additional eutectic-forming ingredients, the meltingpoint of such eutectic being above 800 Fahr.

3. As a net product, an alloy containing magnesium in excess of per cent, zinc between 0.30 and 0.65 per cent, together with aluminum as an additional, eutectic-forming ingredient, the melting point of such eutectic being above 800 Fahr.

4. As a new product, an alloy containing magnesium in excess of 80\per cent, zinc between 0.30 andv 0.65 per cent, together with aluminum and copper as additional, eutec- 6. As a new product, an alloy containing 10 tic-forming ingredients, the melting point of approximately 88 percent of magnesium, such eutectic being above 800 Fahr. 8.5 per cent of aluminum, 2 per cent of cop- 5. As a new product, an alloy containing per, 1 per cent of cadmium and not to exceed magnesium in excess of 80 per cent, zinc be- 0.5 per cent of zinc.

tween 0.30 and 0.65 per cent, together with Signed by us, this/2nd day of December, '15

aluminum, copper and cadmium, each in 1921. consequential amount but aggregating less HERBERT H. DOW. than 20 per cent. J OHN A. GAN N. 

